1. Field of the Invention
This invention relates to gaseous fuel-fired heating appliances and venting systems for such appliances. More particularly, this invention relates to gaseous fuel-fired heating appliances, such as residential furnaces, and gaseous fuel-fired water heating appliances, such as residential water heaters, both of which can be vented through a common, side-wall, PVC vent.
2. Description of Prior Art
Conventional gas heating appliances, such as furnaces, boilers, and water heaters provide the user with safe, economical space and water heating, all the while requiring little maintenance over a relatively long appliance lifetime. These appliances (particularly mid-efficiency, non-condensing configurations) typically use single wall galvanized vent connectors and either a masonry chimney or Type-B vent pipe to vent the flue gases generated by the combustion process during operation of the appliances. The American National Standards Institute (ANSI) categorizes gas appliances based on the vent pressure produced in a special test vent and the difference between the actual temperature and the dew point temperature of the flue gas.
A conventional Category I space heating appliance is one which has a vertical vent which operates under negative static vent pressure with a minimum of condensation. Moisture normally does not condense from the flue gas in Category I appliances because the actual flue gas temperature is generally higher than 140.degree. F. above its dew point temperature. Conventional draft hood equipped appliances are Category I appliances as well as most mid-efficiency, fan-assisted appliances. Mid-efficiency, fan-assisted appliances differ from conventional draft hood appliances by having an induced-draft blower to draw the combustion gases through a heat exchanger and discharge them into a vent. These appliances are classified as Category I appliances if the flue gas temperature is in the same range as the conventional Category I appliance, and if the induced-draft blower and the vent system are designed to maintain a negative pressure in the vent. Venting systems for Category I appliances typically include Type-B vents, lined masonry chimneys, and single wall metal vents.
Category III appliances operate with a positive vent pressure used with a vent gas temperature generally at least 140.degree. F. above its dew point temperature. Because the pressure in the vent is greater than the pressure of the surrounding atmosphere, these appliances require an airtight vent to prevent leakage of flue gases into the residence. An example of a Category III appliance is a mid-efficiency furnace that is vented horizontally through the side-wall of the residence. Venting systems for Category III appliances typically include high temperature plastic and single wall stainless steel metal vents.
Category IV gas heating appliances operate with a positive vent pressure and at a vent-gas temperature less than 140.degree. F. above the dew point temperature. Because the pressure in the vent exceeds that of the surrounding atmosphere and because condensation occurs in the vent, these appliances require an airtight, corrosion-resistant vent that is equipped for condensate disposal. Venting systems for Category IV appliances typically include polyvinylchloride (PVC) or chlorinated polyvinylchloride (CPVC) vents.
From the above discussion it can be seen that the category to which a particular appliance is assigned determines the installation requirements of the venting system for the particular appliance. For example, a Category I appliance may utilize traditional venting materials such as Type-B vent pipe or a masonry chimney, while a Category IV appliance will require an air-tight vent system built from corrosion resistant materials.
The flue gases of gas heating appliances, such as furnaces and water heaters, contain a large amount of water vapor. Because the industry has moved to higher efficiency appliances, and, subsequently, to lower flue gas temperatures, condensation of water and corrosive substances from the flue gases onto vent system surfaces is a major design issue, the consequences of which include the requirement that a building owner with such gas appliances may be required to undergo an expensive and time-consuming vent system replacement.
In an attempt to avoid these costs, several manufacturers have designed appliances with draft hoods that entrain dilution air into the vents. Entraining dilution air into the vents reduces the amount of condensation formed during operation, thereby reducing the number of installations which would require vent system modifications, such as chimney relining. Unfortunately, this process also allows heated room air to escape in an uncontrolled fashion, both while the appliance is operating and while the appliance is idle. The escaping heat increases the heat load on the building, thereby increasing the energy cost associated with controlling the building temperature. In addition, typical draft hood equipped appliances are susceptible to backdrafting, a particularly troublesome problem in multi-story houses.
In buildings provided with gaseous fuel-fired water heaters and gaseous fuel-fired furnaces, the water heater is frequently vented in parallel with the furnace through a common venting system for Category I appliances including Type-B vent, lined masonry chimney, and single wall metal vent. However, if the furnace is replaced with a furnace suitable for venting through a side-wall venting system, the water heater may no longer by itself be compatible with the venting system of the furnace, resulting in an expensive repair or replacement of the water heater vent system or replacement of the gaseous fuel-fired water heater with an electric water heater. Or, if in new construction the gaseous fuel-fired water heater is the only appliance requiring a vertical vent, the choice of an electric water heater becomes more attractive for first cost reason (even though life-cycle cost of electric water heating may be much more due to energy cost differences between electricity and gas).